In a machine for continuous casting of metallurgical products, secondary cooling of the product is conventionally provided by ramps of jets which spray a cooling liquid, generally water optionally mixed with air, onto the product. Spraying of the product commences immediately below the ingot mold and can be continued until the product reaches the bending and extraction zone. However, most often, spraying is interrupted before the straightening zone.
Today, it is known that the final quality of the cast product is greatly influenced by the manner in which its secondary cooling has been conducted Good adjustment of the latter makes it possible, in particular
to ensure complete solidification of the product before its straightening or its oxygen cutting; PA1 to ensure good mechanical behavior of the solidified skin along the machine and, in particular, to avoid the problems of bulging due to too high a surface temperature which can generate internal cracks and considerable central segregation; PA1 to ensure a certain uniformity in the cooling of the product and to avoid sudden reheating or cooling capable of creating cracks at the solidification front (internal cracks) or surface cracks; PA1 to maintain the surface temperature on straightening in the zone of good forgeability of the metal and to thus avoid the formation of transverse cracks on the undersurface. PA1 a determination is made, by means of the speed of casting, of the time t.sub.o at which commences, at the upper part of the ingot mold, the portion of product which, at the time t.sub.vo, will reach the point HD, PA1 a determination is made of the times t.sub.1,...t.sub.1,...t.sub.n at which the portion of product commencing at t.sub.o will emerge from the zones 1,...i,...n of the secondary cooling, PA1 from the time t.sub.1, a flow of cooling fluid adapted to the compensation for the said change in temperature is imposed in the zone i, PA1 and, from the time t.sub.vo, the cooling method conventionally used on the machine is recommenced, varying according to the actual speed of casting.
Secondary cooling is conventionally divided into various successive spraying zones along the cast product. Within each of these zones, the flow of water can be adjusted independently of the other zones The production of a good-quality product is connected to a correct definition of the flows of water in the various zones, particularly in relation to the speed of casting, i.e., the speed of extraction of the product from the machine.
When the speed of casting is constant, the definition of a suitable secondary cooling method poses no problems. In the case of a small variation in the speed of casting, even if sudden, the cooling of the product deviates only relatively slightly from the ideal program defined for a continuous operation and the quality of the product is scarcely affected thereby
This does not apply when the advance of the product passes through a major transition, corresponding to an increase or, above all, to a sudden and considerable reduction in the speed of casting, or even to stoppage of extraction.
When such a transition occurs, the product present in the machine has its cooling disrupted relative to the ideal foreseen program. This disruption particularly affects the portion of the product which, during this transition, is passing through the zone of the machine located between the end of secondary cooling and the straightening point. In this zone, the product cools naturally, particularly by means of radiation, without being sprayed. The transition in speed has the effect of modifying the residence time of the product in this zone of natural cooling. As the operators are no longer able to control the speed of cooling of this portion of the product, this portion reaches the straightening point at a temperature which is substantially different from that which it would have had if the speed of casting had remained normal. This phenomenon is particularly damaging when the speed of casting becomes low or zero during the transition. In fact, under these conditions, cooling of the product is accentuated and the latter reaches the straightening point at a temperature which risks being too low because it is situated outside the zone of good forgeability of the metal.
Such transitional phases occur unexpectedly when there are incidents connected with the operation of the machine. However, most frequently (in approximately 90% of the cases), they are connected with conventional and foreseeable operations, such as the completion of pouring, or a change of distributor.